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16 Section I: Diagnostics and Planning

and are designed to be used as monocortical screws in long bones
with thick cortices [33].

Bone Plates
In the recent years, there has been a significant paradigm shift in
the way fractures are approached and treated. In orthopedic sur-
gery, the need for perfect reconstruction and absolute stability have
been progressively supplanted by indirect reduction and relative
stability, and a much stronger emphasis is now placed on the pres-
ervation of the fracture biology [34,35]. Associated with this shift,
recent years have seen an explosion of new technologies, plates and
implant designs in both human and veterinary surgery. Bone plates
now come in a variety of materials and shapes and as such we are no
longer limited to the traditional straight rectangular plates used in
the past for the fixation of long bones.

Figure 2.9 Coverage of an extensive craniectomy defect using mesh and Nonlocking Plates
self‐drilling screws. Source: Courtesy of Dr. Michelle Oblak. Traditional, nonlocking plates use the screws to generate compres-
sion of the plate against the bone. The friction generated between
Locking Screws the plate and the bone provides stability. If at any time during activ-
The invention of the locking plate system has led to the parallel ity the forces acting on the fracture exceed the frictional forces gen-
development of locking screws. When a locking screw is tightened, erated by the screws, toggling and pull‐out of the screws will occur
its head engages the plate and effectively locks the screw into the resulting in loss of reduction [32,35]. To minimize this risk of fail-
plate. Although some locking plates allow the surgeon to determine ure, the traditional bone screws are designed with a large thread
the screw insertion angle, once locked, the angle of the screw rela- and a relatively small core diameter to provide maximum pull‐out
tive to the plate cannot be modified and acts as an angle‐stable con- strength and generate maximal compression (Figure 2.11).
struct similar to an external fixator. Unlike traditional screws that To generate adequate friction, traditional bone plates require
have been designed to resist pull‐out forces, locking screws are intimate contact with the bone cortex. This requires a time‐
mostly subjected to bending and shear forces and have been consuming and sometimes difficult process of contouring the plate
designed to resist such forces [32]. Their core diameter has been to match the bone surface. The large area of contact between the
increased to better resist bending and shear forces, while their plate and the bone causes devascularization and osteonecrosis of
thread has become finer and symmetrical to equally resist pull‐out the bone cortex underneath the plate and may predispose the bone
or push‐in forces (Figure 2.10). The head of the screw has also been to delayed healing, refracture, or infection [35–37]. To minimize
redesigned with a conical shape or a thread to provide a locking damage to the bone, bone plates have progressively evolved to limit
mechanism in conjunction with the plate hole. Locking screws are the contact between the plate and the bone [e.g., Limited Contact
generally self‐tapping. Some locking screws have a self‐drilling tip Dynamic Compression Plate (LC‐DCP™), DePuy Synthes, West
Chester, PA, USA], while still optimizing stability. This evolution
has led to the recent development of the locking plate [35].
Scew type and diameter
Locking Plates
3.5 mm 3.5 mm 4.0 mm The search for a bone plate that minimizes the biological impact
Locking Cortical Cancellous
while providing superior stability has led to the development of lock-
ing plate systems. These plates provide a mechanism to lock the
screws within the plate, providing an angle‐stable construct, in ways
similar to external fixators or pin and PMMA constructs. Unlike tra-
ditional plates, locking plates do not require intimate contact with the
bone and the screws do not need to generate compression to provide
stability. In contrast with traditional screws designed to resist pull‐out
forces, locking screws are mostly loaded in shear and bending and are
designed to better fulfil this role (see Figure 2.11). The thread has
become symmetrical to equally resist pull‐out or push‐in forces. The
Core core diameter of the screw has become relatively larger, along with a
diameter 2.8x 3.84 mm 2.5x2.44mm 2.0x1 mm smaller thread to resist bending and shear forces, which are now the
predominant forces acting upon them [32].
Two broad categories of locking plates exist: fixed‐angle and vari-
Figure 2.10 (Left to right) A 3.5‐mm locking screw, 3.5‐mm cortical screw, able‐angle locking plates [32]. In most veterinary implants, locking
and 4.0‐mm cancellous screw. Note the difference in core diameter of the
screws and the relative increase in bending stiffness compared with the can- of the screws is achieved when the threaded screw head engages
cellous screw. Despite its overall larger diameter, the 4.0‐mm cancellous and locks into a corresponding thread cut into the plate hole
screw is nearly four times weaker in bending than the 3.5‐mm locking (Figure 2.12). In contrast, some systems achieve locking when the
screw. smooth conical screw head is press fitted into a softer bushing.